This invention is related generally to an optical device that can switch the direction of impinging light. More particularly, the present invention is related to a holographic optical switch containing polymer dispersed liquid crystals in liquid crystal displays and other electronics.
Liquid crystal display devices (LCD) have many advantages over other types of displays. LCDs provide high picture quality in a small volume and are lightweight. In addition, LCDs have comparatively low power consumption, which is dependent on the type of LCD. This has caused the portable electronic market to focus on LCD use for such applications as small portable televisions, mobile telephones and other communication products, video recording units, notebook computers, and desktop monitors.
Active LCDs, which are the most common LCDs in use, contain substrates, a liquid crystal layer through which light passes, and a pixel electrode on one of the substrates that supplies an electric field to liquid crystal layer to form a light guide panel. The metal used to fabricate the pixel electrode depends on the type of LCD used. Reflective LCDs use a natural or artificial light source located outside the LCD and thus the material used for the pixel electrodes has to be a reflective conductive material such as metal aluminum. However, if the external light intensity is not strong enough, the image displayed by the reflective LCD is poor.
To combat this problem, an intrinsic light source generally called a backlight is added to the liquid guide panel. The backlight is supplied to the liquid crystal layer from a fluorescent lamp, light emitting diode (LED), or electroluminescent diode (EL). The backlight is located behind the display requiring the material used for the pixel electrodes has to be a transparent conductive material such as indium tin oxide (ITO). However, besides increasing the size of the LCD (and adding weight and cost), the backlight also consumes the most power in the LCD, severely decreasing battery life if constantly used.
Decreasing the power consumption has thus been a prominent source of research, leading to the developments of the transflective LCD. In transflective LCDs, the pixel electrodes are usually a combination of aluminum in one location and ITO in another location. This permits external light to be used as the light source when the external light is of high enough intensity to provide a good image and the backlight to be used as the light source when the external light is not of high enough intensity. However, the area in which images can be displayed is reduced in both the transmittance and reflectance modes in a transflective LCD.
Polymer dispersed liquid crystal (PDLC) layers have been used for switching in optical fiber communication applications. In a few applications, PDLC layers have been used in transflective LCDs to increase the display area. PDLC is a photoelectric material that transmits light through the material when a voltage is applied to the structure and renders the structure relatively opaque by scattering the incident light when no voltage is applied. PDLC is a mixture of monomers or oligomers with liquid crystal molecules, and then polymerizing the monomers/oligomers to form a polymer. The liquid crystal molecules aggregate to form micro-droplets and are dispersed in the polymer matrix under certain conditions. The PDLC layer is placed directly below the liquid crystal panel between the backlight and the liquid crystal panel so that, in the reflectance mode when no voltage is applied to the PDLC layer external light is scattered and the liquid crystal display is illuminated and, in the transmission mode when a voltage is applied to the PDLC layer, the PDLC is transparent and allows light from the backlight (which extends over the surface area) directly there beneath to illuminate the liquid crystal layer.
In other transflective LCDs, however, the backlight not directly below the liquid crystal panel. Instead, the backlight is adjacent to a light guide layer beneath the liquid crystal layer. The light guide layer guides light into the liquid crystal layer. PDLC layers have not been used with these structures. In addition, with the advent of more complicated technologies, there exist additional uses for light sources incorporated within transflective LCDs. One readily apparent example is the use of an internal light source to illuminate an area in front of the electronic device without the need for an additional light source. More particularly, for cellular telephones having cameras, the backlight can be used, for example, to illuminate a subject to be photographed and enable a viable picture or series of pictures to be taken. However, no structure or optical switch exists that provides this ability within the required tolerances of the small electronics, e.g. cost, size, weight, ruggedness, and minimal power consumption.